refactor(core): remove legacy instrumentation and finalize adapter architecture (#15)

[joncrangle]

Closes #15

Summary

Removes remaining legacy instrumentation, wrapper modules, and compatibility infrastructure across itchy.nvim, completing the migration to the unified adapter/event architecture.

Changes

• Legacy infrastructure removal:
  • Deleted lua/itchy/adapters/legacy.lua, lua/itchy/wrappers/, and tests/legacy_adapter_spec.lua.
  • Removed dosbatch runtime and deleted its orphaned test fixture tests/test_files/cmd.bat.
• Strict adapter resolution:
  • adapters.resolve now fails explicitly when a runtime specifies an unknown or missing adapter.
  • Removed silent fallbacks to legacy adapters.
  • All built-in runtimes explicitly declare an adapter; custom runtimes provide a registered adapter name or adapter table.
• Dead code removal:
  • Deleted zero-caller helpers from lua/itchy/utils.lua: should_filter_line, get_wrapped_code, parse_line_output, parse_error_output, process_output_text, process_output, process_error, apply_extmarks, is_headless.
• Lua and Snacks integration:
  • Preserved Lua delegation to Snacks.debug.run() without adapter resolution or premature runtime loading.
  • Preserved scratch buffer clear key configuration and snacks_debug namespace clearing.
  • Added safe configuration handling for { enabled = false } tables.
• Documentation and comments:
  • Removed stale #\d+ issue numbers and migration/transitional phrasing from production comments.
  • Rewrote README.md to document adapter-based custom runtimes, removing wrapper hack explanations. Left generated vimdocs in doc/ untouched.
• Test coverage:
  • Added tests/adapters_spec.lua covering strict registry resolution, repository audits against legacy imports/issue references, and Lua/Snacks integration semantics.

Verification

Ran the complete test suite:

nvim -l tests/minit.lua --minitest

Result: 216 test cases passed across 17 test groups (0 fails, 0 notes).